1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory, more particularly to a NAND flash memory.
2. Description of the Related Art
Known as one type of a nonvolatile semiconductor memory is the NAND flash memory. The NAND flash memory has a transfer MOS transistor that transfers a transfer voltage to the word lines.
To write data to the memory cells, the gate voltage is applied to the gates of the transfer MOS transistor and the transfer voltage is transferred to the word lines via the transfer MOS transistor. The gate voltage and transfer voltage are generated by a voltage generating circuit.
The voltage generating circuit has a voltage step-up circuit. The voltage step-up circuit increases the power-supply voltage, generating the gate voltage to be applied to the gate of the transfer MOS transistor. The gate voltage increased by the voltage step-up circuit is decreased by a voltage step-down circuit, thereby providing a transfer voltage to be transferred to the word lines.
The gate voltage to be applied to the gates of the transfer MOS transistor to transfer the transfer voltage to the word lines should be greater than the sum of the transfer voltage and the threshold voltage of the transfer MOS transistor. Note that the gate voltage for the transfer MOS transistor is equal to the sum of the transfer voltage to be transferred to the word lines and the voltage drop achieved by the voltage step-down circuit.
Hence, to transfer the transfer voltage to the word lines, the voltage drop that the voltage step-down circuit achieve must be greater than or equal to the threshold voltage of the transfer MOS transistor.